In a typical electrophotographic method, a latent image-bearing member formed of a photoconductive photoreceptor is charged by corona charging. The charged latent image-bearing member is exposed with a laser or a light-emitting diode (LED) to form an electrostatic latent image. The electrostatic latent image is subjected to reversal development for visualization, thereby forming a high-quality image.
Typically for this method, the latent image-bearing member includes a photosensitive layer composed of an organic photoconductor (OPC). To improve the durability of an image-forming apparatus, the use of a latent image-bearing member including a photosensitive layer composed of amorphous silicon has recently been studied. The latent image-bearing member is abraded by scraping against a printing medium and an elastic blade described below. However, amorphous silicon has excellent abrasion resistance. Thus, the use of the latent image-bearing member including the photosensitive layer composed of amorphous silicon provides a highly durable image-forming apparatus. Specifically, the rate of thickness reduction of amorphous silicon by abrasion is 1/100 or less of those of organic photoconductors.
Furthermore, in the latent image-bearing member including the photosensitive layer composed of amorphous silicon, a reduction in the thickness of the photosensitive layer has been promoted because of its good productivity and better resolution of an image than that in the case of greater thickness.
After the toner image is transferred to a surface of a printing medium, such as paper, toner left on the surface of the latent image-bearing member including the photosensitive layer composed of amorphous silicon is removed by a cleaning device. As the cleaning device, an elastic blade is widely used because it has a simple structure with a small number of moving parts and contributes to the miniaturization of an image-forming apparatus.
Thus, there have been advances in the use of an image-forming apparatus including a latent image-bearing member in combination with a cleaning device having an elastic blade, the latent image-bearing member having a thin photosensitive layer composed of amorphous silicon.
However, in the situation when the latent image-bearing member including the photosensitive layer composed of amorphous silicon is used and the latent image-bearing member is charged by a charging device, corona products are attached to a surface of the latent image-bearing device. In the situation when an image-forming apparatus including the latent image-bearing member in which the corona products are accumulated on the surface of the latent image-bearing member is used, and an electrostatic latent image is formed in a high-temperature and high-humidity environment, the electrostatic latent image on the surface of the latent image-bearing member may be disturbed. This phenomenon is called an image flow.
To inhibit the occurrence of the image flow, a method is reported in which a toner including an external additive, such as titanium oxide, which has a polishing effect, attached to surfaces of particles of the toner is used, and the toner is pressed against the surface of the latent image-bearing member with the elastic blade to remove corona products on the surface of the latent image-bearing member.
As a toner to inhibit the occurrence of the image flow in the image-forming apparatus including the latent image-bearing member having the photosensitive layer composed of amorphous silicon, a toner including a conductive fine particle as an external additive, such as titanium oxide, externally added to a toner base particle thereof, has been used. The conductive fine particle has an average primary particle diameter of 90 nm or less, an average aggregated particle diameter of 0.5 μm to 2.0 μm, and a BET specific surface area of 15 m2/g or more.
However, even if the elastic blade is used, toner left on the surface of the latent image-bearing member is not completely removed. For example, toner particles, magnetic powder, resin pieces, and an external additive, such as silica, accumulate on a distal end portion of the elastic blade, the distal end portion being a portion where the elastic blade is pressed against the latent image-bearing member. When the residue accumulated on the surface of the latent image-bearing member near the elastic blade continues to rub against the elastic blade and the photosensitive layer composed of amorphous silicon for a prolonged period, a charge-up phenomenon occurs in which the residue is excessively charged to a level equal to or larger than an allowable amount of charge.
When the charge-up phenomenon occurs, the amount of charge of the residue exceeds the withstand voltage of the photosensitive layer composed of amorphous silicon, electrical discharge (leakage phenomenon) to a submicroscopic region of the surface of the latent image-bearing member is liable to occur. Thus, in the thin photosensitive layer which is composed of amorphous silicon and which has a low dielectric breakdown voltage, the leakage phenomenon is liable to cause dielectric breakdown. As described above, in the image-forming apparatus including the cleaning device having the elastic blade and the latent image-bearing member having the thin photosensitive layer composed of amorphous silicon, the dielectric breakdown of the photosensitive layer may cause an unrecoverable defect on the surface of the latent image-bearing member. In the situation when the dielectric breakdown of the photosensitive layer occurs in the latent image-bearing member, an image defect, called a “black spot”, may be caused in a formed image.
In the latent image-bearing member including the thin photosensitive layer composed of amorphous silicon as described above, the supply of a conductive external additive, such as titanium oxide, to the surface of the latent image-bearing member in a state in which the external additive is freed from a toner is effective in inhibiting the occurrence of the dielectric breakdown of the photosensitive layer. The employment of such a method disperses the charge of the residue, thereby inhibiting the occurrence of the leakage phenomenon.
However, even if the foregoing toner is used, although the occurrence of the image flow is inhibited, the degree of inhibition is not sufficient, so further improvement is required. Furthermore, even if the foregoing toner is used, in the situation when images with low coverage rate are formed for a long time and where the toner is stirred in a developing unit for a long time, the image density of the formed images may be lower than a desired value. Moreover, in the situation when the foregoing toner is used, the occurrence of the dielectric breakdown of the latent image-bearing member including the thin photosensitive layer composed of amorphous silicon is not necessarily inhibited. In this regard, further improvement is also required.